The present invention relates to a transconductance amplifier suitable for a low-power LSI used in a portable radio equipment, to a filter formed in the LSI with using the transconductance amplifier, and to a transconductance amplifier tuning circuitry used for tuning a gain of the transconductance amplifier in the filter.
In connection with recent wide spread of portable radio equipments, these equipments have been required to be smaller in size and lower in manufacturing cost. In order to satisfy these requirements, important is to reduce the mounting area and also the mounting cost of LSI on the portable radio equipment by using an on-chip filter in stead of a conventional filter with an outboard element. Such on-chip filter might be formed by combining transconductance amplifiers and capacitors.
A conventional transconductance amplifier used for the on-chip filter is configured by a series circuitry, connected between the voltage source and the ground, of a differential input circuit consisting of a pair of transistors for voltage-current conversion of input voltage, a drain voltage adjustment circuit for fixing the drain voltage of the pair of the transistors to a control voltage (voltage for determining the conductance Gm) applied to its control terminal, a current mirror circuit and an output stage.
According to the conventional transconductance amplifier, required is 0.2-0.3 V of the source-drain voltage of the transistors of the differential input circuit to operate these transistors at its linear response region. Also, 0.2-0.3 V is required as for a range of the control voltage of the conductance Gm and furthermore 0.5-0.6 V of the source-drain voltage of an output transistor in the current mirror circuit is required to saturate the output transistor so as to sufficiently increase the output impedance. Therefore, in order to secure an output dynamic range of 0.4-0.5 V or more, it is necessary to keep the source voltage VDD at 1.5 V or more.
It is therefore an object of the present invention to provide a transconductance amplifier which can be operated at a lower source voltage such as 1 V or less for example with keeping a sufficiently high output impedance and a sufficiently wide output dynamic range, to provide a filter using the transconductance amplifier and to provide a transconductance amplifier tuning circuitry for tuning a gain of the transconductance amplifier in the filter.
Another object of the present invention is to provide a small power transconductance amplifier tuning circuitry using the transconductance amplifier.
According to the present invention, a transconductance amplifier has an input stage and an output stage. The input stage includes a differential input circuit for converting a differential voltage signal applied thereto into a differential current signal, a first pair of regulated cascode circuits for adjusting output voltages of the differential input circuit depending upon a control voltage applied thereto, and an input section of a pair of current mirror circuits for mirroring the differential current signal from the differential input circuit. The differential input circuit, the first pair of regulated cascode circuits, and the input section of the pair of current mirror circuits are connected in series with each other between a voltage source and a ground. The output stage includes an output section of the pair of current mirror circuits, a differential current source circuit, a second pair of regulated cascode circuits for keeping output voltages of the current mirror circuit to a first bias voltage applied thereto, a third pair of regulated cascode circuits for keeping output voltages of the differential current source circuit to a second bias voltage applied thereto, output terminals of the third pair of regulated cascode circuits being connected to output terminals of the second pair of regulated cascode circuits, and a pair of amplifier output terminals connected to the output terminals of the second and third pairs of regulated cascode circuits. The output section of the pair of current mirror circuits, the second and third pairs of regulated cascode circuits and the differential current source circuit are connected in series with each other between the voltage source and the ground.
The input stage for adjusting the gain or conductance Gm depending upon the control voltage applied and the output stage for securing a sufficiently high output impedance and a sufficiently wide output dynamic range are connected between the voltage source and the ground in parallel with each other. Thus, the gain can be adjusted over a wider range even if the source voltage is lower than the conventional one, for example at 1 V or less. Also, a sufficiently high output impedance and a sufficiently wide output dynamic range can be expected at the lower source voltage.
It is preferred that the transconductance amplifier further includes a feedback circuit for common-mode noise suppression. This feedback circuit controls, depending upon common-mode noise applied to the transconductance amplifier, output voltages at the pair of amplifier output terminals to a predetermined voltage.
In this case, it is more preferred that the differential current source circuit controls currents flowing through the second and third pairs of regulated cascode circuits depending upon a feedback signal provided from the feedback circuit.
It is also preferred that the differential input circuit consists of two enhancement MOS transistors with a low threshold voltage, for receiving the differential voltage signal applied thereto, respectively. In this case, more preferably, the low threshold voltage of the enhancement MOS transistors is less than 0.2 V. As a consequence of this configuration, the source voltage can be more lowered.
It is preferred that the differential input circuit consists of two depletion MOS transistors for receiving the differential voltage signal applied thereto, respectively. As a consequence of this configuration, the source voltage can be further lowered.
It is also preferred that the differential input circuit consists of first and second transistors with gates connected respectively to first and second amplifier input terminals and with sources connected together to the ground or the voltage source.
It is preferred that each pair of the first, second and third pairs of regulated cascode circuit consists of an operational amplifier and a transistor with a gate connected to an output terminal of the operational amplifier.
It is also preferred that the first pair of regulated cascode circuits consist of third and fourth transistors with sources connected respectively to output terminals of the differential input circuit, a first operational amplifier with a non-inverting input terminal connected to a control terminal which receives the control voltage for controlling a conductance Gm, with an inverting input terminal connected to a source of the third transistor and with an output terminal connected to a gate of the third transistor, and a second operational amplifier with a non-inverting input terminal connected to the control terminal, with an inverting input terminal connected to a source of the fourth transistor and with an output terminal connected to a gate of the fourth transistor.
It is further preferred that the second pair of regulated cascode circuits consist of ninth and tenth transistors with sources connected respectively to output terminals of the current mirror circuit, a third operational amplifier with a non-inverting input terminal connected to a first bias terminal, with an inverting input terminal connected to a source of the ninth transistor and with an output terminal connected to a gate of the ninth transistor, and a fourth operational amplifier with a non-inverting input terminal connected to the first bias terminal, with an inverting input terminal connected to a source of the tenth transistor and with an output terminal connected to a gate of the tenth transistor.
It is also preferred that the third pair of regulated cascode circuits consist of thirteenth and fourteenth transistors with sources connected respectively to output terminals of the differential current source circuit, a fifth operational amplifier with a non-inverting input terminal connected to a second bias terminal, with an inverting input terminal connected to a source of the thirteenth transistor and with an output terminal connected to a gate of the thirteenth transistor, and a sixth operational amplifier with a non-inverting input terminal connected to the second bias terminal, with an inverting input terminal connected to a source of the fourteenth transistor and with an output terminal connected to a gate of the fourteenth transistor.
It is preferred that the first bias voltage (Vb1) is set to a voltage of about xc2xe of a source voltage or more and the second bias voltage (Vb2) is set to a voltage of about xc2xc of the source voltage or less, or that the first bias voltage (Vb1xe2x80x2) is set to a voltage of about xc2xc of the source voltage or less and the second bias voltage (Vb2xe2x80x2) is set to a voltage of about xc2xe of a source voltage or more. Thus, a sufficient output dynamic range can be secured.
According to the present invention, also, a filter is provided with a plurality of transconductance amplifiers each having the aforementioned configuration.
According to the present invention, further, a transconductance amplifier tuning circuitry includes two reference signal input terminals to which reference frequency signals are input, an RC phase-shifter connected to the two reference signal input terminals and provided with two transconductance amplifiers each having the aforementioned configuration and two capacitors, a multiplier with input terminals connected to two output terminals of the RC phase-shifter and to the two reference signal input terminals, and an operational amplifier with two differential input terminals connected to two output terminals of the multiplier and with an output terminal connected to control terminals of the transconductance amplifiers. The tuning circuitry feedback-controls gains of the two transconductance amplifiers so that the RC phase-shifter always shifts a phase of the input reference frequency signals by 90 degrees.
In the conventional transconductance amplifier tuning circuitry input reference frequency signals are applied to a RC low pass filter and a CR high pass filter configured by four transconductance amplifiers and capacitors, and the gains of these transconductance amplifiers are feedback controlled so that output signal amplitudes of these filters become equal to each other. Thus, required is four transconductance amplifiers causing large power consumption and increased occupying area in a LSI chip.
Whereas, since the tuning circuitry according to the present invention is configured by the RC phase-shifter for shifting the input reference frequency signal phase by 90 degrees, the multiplier and the operational amplifier, required is only two transconductance amplifiers and two capacitors resulting smaller power consumption and smaller occupying area in a LSI chip.
It is preferred that the RC phase-shifter includes a first transconductance amplifier with first and second input terminals connected to the two reference signal input terminals, a first capacitor connected between the first input terminal of the first transconductance amplifier and a first output terminal of the first transconductance amplifier, a second capacitor connected between the second input terminal of the first transconductance amplifier and a second output terminal of the first transconductance amplifier, and a second transconductance amplifier with a first output terminal connected to the second output terminal of the first transconductance amplifier and with a second output terminal connected to the first output terminal of the first transconductance amplifier. A first input terminal of the second transconductance amplifier is connected to the first output terminal of the second transconductance amplifier and a second input terminal of the second transconductance amplifier is connected to the second output terminal of the second transconductance amplifier.
It is also preferred that the multiplier is a mixer circuit with first two input terminals connected to the two reference signal input terminals and with second two input terminals connected to the first and second output terminals of the first transconductance amplifier.
It is preferred that the circuitry further includes a control signal output terminal connected to the output terminal of the operational amplifier, for outputting the control signal outside.
It is also preferred that the circuitry further includes a third capacitor for smoothing, connected between the control signal output terminal and the ground.
According to the present invention, a transconductance amplifier includes first and second transistors with gates connected respectively to first and second amplifier input terminals to which a differential voltage is input and with sources connected together to a first voltage source, third and fourth transistors with sources connected respectively to drains of the first and second transistors, a first operational amplifier with a non-inverting input terminal connected to a control terminal which receives a signal for controlling a conductance Gm, with an inverting input terminal connected to a source of the third transistor and with an output terminal connected to a gate of the third transistor, a second operational amplifier with a non-inverting input terminal connected to the control terminal, with an inverting input terminal connected to a source of the fourth transistor and with an output terminal connected to a gate of the fourth transistor, fifth and sixth transistors with sources connected together to a second voltage source and with drains and gates connected respectively to drains of the third and fourth transistors, seventh and eighth transistors with sources connected together to the second voltage source and with gates connected respectively to the drains and gates of the fifth and sixth transistors, ninth and tenth transistors with sources connected respectively to drains of the seventh and eighth transistors, a third operational amplifier with a non-inverting input terminal connected to a first bias terminal, with an inverting input terminal connected to a source of the ninth transistor and with an output terminal connected to a gate of the ninth transistor, a fourth operational amplifier with a non-inverting input terminal connected to the first bias terminal, with an inverting input terminal connected to a source of the tenth transistor and with an output terminal connected to a gate of the tenth transistor, eleventh and twelfth transistors with sources connected together to the first voltage source, thirteenth and fourteenth transistors with drains connected respectively to drains of the ninth and tenth transistors and with sources connected respectively to drains of the eleventh and twelfth transistors, a fifth operational amplifier with a non-inverting input terminal connected to a second bias terminal, with an inverting input terminal connected to a source of the thirteenth transistor and with an output terminal connected to a gate of the thirteenth transistor, a sixth operational amplifier with a non-inverting input terminal connected to the second bias terminal, with an inverting input terminal connected to a source of the fourteenth transistor and with an output terminal connected to a gate of the fourteenth transistor, and first and second amplifier output terminals connected to the drains of the ninth and thirteenth transistors and the drains of the tenth and fourteenth transistors, respectively.
The input stage for adjusting the gain or conductance Gm depending upon the control voltage applied (first to sixth transistors and first and second operational amplifiers) and the output stage for securing a sufficiently high output impedance and a sufficiently wide output dynamic range (seventh to fourteenth transistors and third and fourth operational amplifiers) are connected between the first and second voltage sources in parallel with each other. Thus, the gain can be adjusted over a wider range even if the source voltage is lower than the conventional one, for example at 1 V or less. Also, a sufficiently high output impedance and a sufficiently wide output dynamic range can be expected at the lower source voltage.
It is preferred that the transconductance amplifier further includes a feedback circuit for common-mode noise suppression. This feedback circuit has input terminals connected to the drains of the fifth and sixth transistors and to the first and second amplifier output terminals, and an output terminal connected to gates of the eleventh and twelfth transistors for providing a common-mode noise canceling signal when signals at its input terminals change due to a common-mode noise occurrence.
In this case, it is also preferred that the feedback circuit has fifteenth and sixteenth transistors with gates connected together to the drain and gate of the sixth transistor and with sources connected together to the second voltage source, seventeenth and eighteenth transistors with gates connected together to the drain and gate of the fifth transistor and with sources connected together to the second voltage source, a nineteenth transistor with a gate connected to the first amplifier output terminal and with a source connected together to drains of the fifteenth and seventeenth transistors, a twentieth transistor with a gate connected to the second amplifier output terminal and with a source connected together to drains of the sixteenth and eighteenth transistors, a twenty-first transistor with a gate connected to a third bias terminal and with a source connected together to the drains of the fifteenth and seventeenth transistors, a twenty-second transistor with a gate connected to the third bias terminal and with a source connected together to the drains of the sixteenth and eighteenth transistors, a twenty-third transistor with a source connected to the first voltage source and with gate and drain connected together to drains of the nineteenth and twentieth transistors, and a twenty-fourth transistor with a source connected to the first voltage source and with gate and drain connected together to drains of the twenty-first and twenty-second transistors and to gates of the eleventh and twelfth transistors.
It is further preferred that the first voltage source is a ground and the second voltage source is a positive voltage source, wherein the first, second, third, fourth, eleventh, twelfth, thirteenth and fourteenth transistors are NMOS transistors, and wherein the fifth, sixth, seventh, eighth, ninth and tenth transistors are PMOS transistors, or that the first voltage source is a positive voltage source and the second voltage source is a ground, wherein the first, second, third, fourth, eleventh, twelfth, thirteenth and fourteenth transistors are PMOS transistors, and wherein the fifth, sixth, seventh, eighth, ninth and tenth transistors are NMOS transistors.
It is preferred that the first and second transistors are enhancement MOS transistors with an absolute threshold voltage less than 0.2 V, or that the first and second transistors are depletion MOS transistors. As a consequence of this configuration, the source voltage can be more lowered.
According to the present invention, a filter is provided with a plurality of transconductance amplifiers each having the aforementioned configuration.
Furthermore, according to the present invention, a transconductance amplifier tuning circuitry includes first and second reference signal input terminals for receiving reference frequency signals, a control signal output terminal for outputting a control signal, first and second transconductance amplifiers each having the aforementioned configuration, first and second capacitors, a mixer circuit and an operational amplifier. The first and second reference signal input terminals are connected to first and second input terminals of the first transconductance amplifier. The first input terminal of the first transconductance amplifier and a first output terminal of the first transconductance amplifier are connected via the first capacitor, the second input terminal of the first transconductance amplifier and a second output terminal of the first transconductance amplifier being connected via the second capacitor. The second output terminal of the first transconductance amplifier is connected to a first output terminal of the second transconductance amplifier, and the first output terminal of the first transconductance amplifier is connected to a second output terminal of the second transconductance amplifier. A first input terminal of the second transconductance amplifier is connected to the first output terminal of the second transconductance amplifier, and a second input terminal of the second transconductance amplifier is connected to the second output terminal of the second transconductance amplifier. First two input terminals of the mixer circuit are connected to the first and second reference signal input terminals, and second two input terminals of the mixer circuit are connected to the first and second output terminals of the first transconductance amplifier. Two output terminals of the mixer circuit are connected respectively to two differential input terminals of the operational amplifier. An output terminal of the operational amplifier is connected to control terminals of the first and second transconductance amplifiers and to the control signal output terminal. An RC phase-shifter is configured by the first and second transconductance amplifiers and the first and second capacitors, the mixer circuit and the operational amplifier to form a feedback loop for controlling gains of the first and second transconductance amplifiers so that the RC phase-shifter always shifts a phase of the input reference frequency signals by 90 degrees.
Since the tuning circuitry according to the present invention is configured by the RC phase-shifter for shifting the input reference frequency signal phase by 90 degrees, the mixer circuit and the operational amplifier, required is only two transconductance amplifiers and two capacitors resulting smaller power consumption and smaller occupying area in a LSI chip.
It is preferred that the circuitry further includes a third capacitor for smoothing, connected between the control signal output terminal and a ground.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.